15
Hydrology

Water is the most important single determinant of the earth's
climate. Water covers 70 percent of the earth's surface. The oceans
store heat, and they absorb CO2 and
other atmospheric chemicals. Snow fields, glaciers, ice sheets, and
sea ice are collectively the greatest mass of fresh water on earth.
They exercise a major influence on the planet's overall albedo
(surface reflectivity). Finally, water vapor is the predominant
greenhouse gas. Water vapor, water droplets, and ice crystals are
crucial elements in the climatic system.

Several aspects of the hydrologic cycle are important with
respect to climate change. As components of the climatic system
begin to warm, other factors come into play that amplify or reduce
the initial warming. Some of these are atmospheric phenomena or
processes directly affecting the earth's radiative balance (e.g.,
water vapor feedbacks and cloud feedbacks). Some are land-based
phenomena or processes with impacts on radiative balance (e.g.,
snow and ice feedbacks and feedbacks involving surface albedo and
temperature, or snow cover and soil moisture). Least
well-understood are phenomena or processes that involve the
biosphere (e.g., evaporation and transpiration). This chapter
describes mechanisms involving the movement of water through the
hydrologic cycle. Related mechanisms whose functions rely more
directly on exchange of energy with the atmosphere are described in
Chapter 12.

Mechanisms Involving Land Surface
Hydrology

Precipitation

Precipitation and soil moisture content, and the resulting
runoff, are important components in the climatic system. One
computer simulation, which examined a shift in ground cover in the
Amazon basin from forest and

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15
Hydrology
Water is the most important single determinant of the earth's
climate. Water covers 70 percent of the earth's surface. The oceans
store heat, and they absorb CO2 and
other atmospheric chemicals. Snow fields, glaciers, ice sheets, and
sea ice are collectively the greatest mass of fresh water on earth.
They exercise a major influence on the planet's overall albedo
(surface reflectivity). Finally, water vapor is the predominant
greenhouse gas. Water vapor, water droplets, and ice crystals are
crucial elements in the climatic system.
Several aspects of the hydrologic cycle are important with
respect to climate change. As components of the climatic system
begin to warm, other factors come into play that amplify or reduce
the initial warming. Some of these are atmospheric phenomena or
processes directly affecting the earth's radiative balance (e.g.,
water vapor feedbacks and cloud feedbacks). Some are land-based
phenomena or processes with impacts on radiative balance (e.g.,
snow and ice feedbacks and feedbacks involving surface albedo and
temperature, or snow cover and soil moisture). Least
well-understood are phenomena or processes that involve the
biosphere (e.g., evaporation and transpiration). This chapter
describes mechanisms involving the movement of water through the
hydrologic cycle. Related mechanisms whose functions rely more
directly on exchange of energy with the atmosphere are described in
Chapter 12.
Mechanisms Involving Land Surface
Hydrology
Precipitation
Precipitation and soil moisture content, and the resulting
runoff, are important components in the climatic system. One
computer simulation, which examined a shift in ground cover in the
Amazon basin from forest and

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Page 136
savannah to pasture, showed regional climate change with a
weakened hydrologic cycle exhibiting reductions in both
precipitation and evaporation (Lean and Warrilow, 1989). A
realistic simulation of precipitation is an important
characteristic for studies of climate change. All general
circulation models (GCMs) simulate broad features of the observed
precipitation pattern, but they also contain significant errors.
These include inadequate characterization of the Southeast Asian
summer monsoon rainfall and the summer rains in the southern Zaire
basin. Recent models also show large differences in estimates of
the intensity of tropical ocean rainbelts (Intergovernmental Panel
on Climate Change, 1990).
Soil Moisture
Soil moisture is the "control valve" of the land surface
hydrology. Soil moisture is the source of water for evaporation and
thus controls heat transfer from the land surface. It also is the
principal absorber of heat in the surface. Precipitation and soil
moisture, and the associated runoff, are directly interconnected.
Soil moisture is an important factor for vegetation, including
agricultural crops, and through them affects evapotranspiration,
surface reflectivity, and other aspects of the climatic system.
General circulation models appear to be quite sensitive to the
proper formulation of the hydrologic budgets of the land surfaces
of the earth. For example, numerical experiments reviewed by Mintz
(1982) have shown that large-scale changes of land surface
evaporation in GCMs produce significant changes in the predicted
circulation and precipitation. Smaller and more realistic soil
moisture anomalies may not produce such drastic changes, but it
appears (e.g., Rowntree and Bolton, 1983) that they can have
considerable impact on the climate of the region surrounding the
anomally. This and other evidence (e.g., Rind, 1982; Shukla and
Mintz, 1982; Sud and Fennessy, 1984; Yeh et al., 1984) indicate
that there is a critical need for sound parametric expressions for
evaporation and related land surface processes over areas with
typical length scales of hundreds of kilometers. While the details
of most processes at local scales are well known and understood, as
of now there is no agreement on how these hydrologic processes
should best be parameterized at the scales appropriate for GCMs.
General circulation models are complex in structure, and they
involve intense and sophisticated computational schemes. Yet, in
most instances their representation of the hydrology of the earth's
land surfaces is crude and not well tested.
The Biosphere
Climate affects ecosystems in a variety of ways. It is an
important influence on processes that determine the carbon and
nutrient cycles of

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ecosystems. It also affects the community structure of
ecosystems. Predicting how ecosystems will respond to climate
change is difficult for at least three reasons. First, ecosystems
contain a complex web of interactions among biological processes.
Direct effects on one process may indirectly influence other
processes in ways we do not yet understand. Second, the response of
ecosystems to specific climatic changes depends in part on what
other environmental factors are changing. A clear example is the
interaction between changes in precipitation and CO2, which functions as a growth stimulant
to green plants. Third, current climate change predictions are not
sufficiently detailed to permit conclusions about the consequences
for the biosphere for either natural ecosystems or land management
practices. For many biological processes, changes in temperature
extremes and the subannual patterns of temperature and moisture are
more important than changes in annual mean values. Thus it is
difficult to assess the interactions between the biosphere and the
climatic system.
Nevertheless, there are some clear interactions. The most
important short-time-scale role of the land biosphere in the
climatic system is its control of evapotranspiration. Any attempt
to develop a realistic land surface parameterization must separate
the functioning of vegetation from that of the soil in the
hydrologic cycle. Vegetation intercepts water (when precipitation
evaporates from leaves before it reaches the soil), extracts water
from the soil through roots, slows the transfer of water from soil
into the atmosphere, alters wind patterns in ways that affect soil
temperature and rates of evaporation or evapotranspiration, and
causes differences in surface reflectivity (albedo).
These and other aspects of vegetation and its effects need to be
characterized in sufficiently simple terms to be incorporated in
GCMs. Adequate data on vegetation cover and soils would also be
required.
Impact of Greenhouse Warming on the
Hydroligic Cycle
The implications of greenhouse warming for changes in the
hydrologic cycle and concomitant water resources were studied by
the Panel on Water and Climate of the National Research Council
(1977). Much of the more recent research in the 1980s has been
reviewed by Gleick (1989). Two major approaches can be
distinguished to analyze the problem, namely, the direct and the
indirect approach.
In the direct approach, hydrologic variables such as runoff and
soil moisture are part of the primary output of a climate
forecasting method. Such climate forecasting can be based on
paleoclimatic records, more recent (historical) records, and GCM
computations. Unfortunately, however, it appears that among all
forecasting variables the specifically hydrologic variables

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are the ones that involve the largest degree of uncertainty. For
example, even when different GCM outputs are in general agreement
on changes in precipitation and temperature, they show much larger
discrepancies in soil moisture and runoff. Moreover, the spatial
scales of the output of the direct forecasting methods are usually
one or more orders of magnitude larger than those of watersheds and
catchments, which are common in hydrology.
Mainly for this reason, most hydrologists concerned with the
likelihood of climate change have made use of indirect approaches.
In this second class of approaches the specifically climatic
outputs of climate forecasting methods are used as inputs for more
detailed hydrologic models, which operate at smaller scales more
appropriate for river basins and hydrologic catchments.
Unfortunately, the state of the art in hydrologic modeling is
probably less advanced than that of the current GCM technology.
Most hydrologic simulation models, relating precipitation, runoff,
and the moisture state of the catchment, are heavily parameterized
and thus require an extensive data record for calibration and
validation. Thus the panel concludes they cannot be trusted when
they are applied to changing conditions outside their range of
calibration. Furthermore, because the outputs from climate
forecasting methods are generally acknowledged to be unreliable, in
many instances these hydrologic models have been run with
hypothetical climate scenarios as input. Usually, there is no way
of knowing how internally consistent or otherwise realistic they
are.
In spite of these uncertainties a few plausible results emerge
from all the research so far. One is that with greenhouse warming,
there is a distinct possibility over the mid-latitudes of a
relative decrease in winter precipitation in the form of snow. This
may in turn result in decreased water storage in the form of winter
snowpack. Earlier melting of this snowpack and increased
evaporation during the summer may then lead to increased aridity in
many areas of the world.
In view of the potentially disastrous consequences of such
scenarios, and because of the need to reduce the extreme
uncertainty surrounding all this work, it is clear that a vigorous
research program is urgently called for. A major effort should be
directed toward a better understanding of the main hydrologic
transport phenomena at scales relevant for process modeling in
climate dynamics and for more regional catchment modeling in
hydrology. An important component of this will be the design and
execution of experiments on these scales.
References
Gleick, P. H. 1989. Climate change, hydrology and water
resources. Reviews of Geophysics 27:329–344.
Intergovernmental Panel on Climate Change. 1990. Climate Change:
The IPCC